In the proposed Universal Mobile Telephony System (UMTS), defined by the standards published by the 3rd Generation Partnership Project (3GPP), a mobile device, or user element UE, can establish links with several cells in the cellular radio network.
The data received by the user equipment from each of the cells is transmitted in frames on respective downlink channels. The frames in the respective downlink channels are not synchronized with each other. Moreover, because of possible movement of the user equipment relative to the base stations in the different cells, the timing of each downlink frame, relative to the other received downlink frames, can change.
The user equipment transmits data in frames on uplink channels to the base stations in the cells with which the user equipment has links. Thus, there is defined for the user equipment a transmission frame timing, which is common to all of the uplink channels. This transmission frame timing is defined with reference to the downlink frame timing of one of the active links.
Thus, in the Technical Specification 3GPP TS 25.133 v3.8.0, section 7, and the associated test case at section A.7, there is described a method for defining the timing of the, uplink transmission frame. Specifically, in the case where there is a single active link, the uplink frame transmission takes place a time T0 (=1,024 chips) after the reception of the first detected path of the corresponding downlink frame.
When the user equipment has one active link with a cell, its transmission frame timing is set with reference to the downlink frame timing of that cell, and the transmission frame timing is still defined with reference to the downlink frame timing of that first cell, even when a second cell is added to the active set. However, in that case, when the first cell is removed from the active set, the uplink transmit timing is adjusted, at a defined adjustment rate, until the uplink frame transmission takes place T0 (=1,024 chips) after the reception of the first detected path of the downlink frame from the remaining active link.
Thus, according to the published Technical Specification, there is defined a reference cell, that is the cell with reference to which the uplink frame transmission is defined. When there are two active links, and the reference cell is removed from the active set, the remaining cell becomes the reference cell.
However, this does not indicate how the uplink frame transmission should be defined in the event of a soft handover, when there are more than two active links, and the reference cell is removed from the active set.